There is great variation in the age when signs of HIV-associated illness first appear in children infected perinatally. In long-term survivors a combination of specific, innate virologic properties and host immune responses keep the infection "in check", as compared to virologic parameters in other infected infants who develop rapidly progressive disease. We will detail the relationships that exist between the biologic characteristics (viral phenotype) displayed by primary pediatric isolates of HIV-1 in vitro and clinical manifestations of disease. We will determine whether sequential isolates become more pathogenic as the infection progresses and whether pediatric long-term survivors have viral isolates which display phenotypes associated with decreased pathogenicity. We will attempt to cultivate all isolates in monocyte-derived macrophages to learn how this growth property evolves and if it correlates with changes in the pace of clinical illness or with CNS disease. We will determine whether there are consistent associations between specific sequences in the V1, V2 and V3 loops of env and viral phenotype. We will also study the degree to which the genotype of virus isolates passaged in culture conforms with the genotype of feral virus strains that are isolated de novo from blood mononuclear cells and plasma. In order to study in greater detail the relationships between genotype and phenotype, infectious proviral DNA clones of a small number of mother and child isolates will be obtained and their biologic behavior examined. Portions of the env region of these clones will be sequenced. We will look for genetic features that are associated with particular phenotypes. In these studies we will make extensive use of an archive of samples (virus stocks, culture supernatant fluids, frozen PBMC, plasma, etc) that have been collected from 70 HIV-1 infected children in Connecticut, including 27 long-term survivors, and of new specimens that we will collect prospectively. Most of our patients are also enrolled in a prospective, longitudinal cohort study; hence, demographic and clinical data are available to define associations between phenotype, genotype and clinical expression of disease.